1. Field of the Invention
The present invention relates to a load control apparatus that controls an operation of a load by controlling power supplied to the load by opening and closing of a relay.
2. Description of the Related Art
It has conventionally been the practice that an operation of a load such as pump, light, etc., mounted to a machine tool is controlled by opening and closing of a relay provided between a power supply and the load. FIG. 4 is a view for explaining an operation of general load control apparatus. A relay control unit 111 outputs an open command or a close command to a relay 2 in accordance with a load control command inputted via manipulation by the operator (user) or inputted from an external device (not illustrated) that operates based on a predetermined program. The relay 2 is provided between a power supply 4 and a load 3. A load control apparatus 101 includes the relay control unit 111. The relay 2 operates such that contacts are closed upon receipt of a close command from the relay control unit ill. When the contacts of the relay 2 are closed, a closed circuit including the relay 2, the power supply 4 and the load 3, is constituted so that power is supplied from the power supply 4 to the load 3. Further, the relay 2 operates such that the contacts are opened upon receipt of an open command from the relay control unit 111. When the contacts of the relay 2 are opened, power is no longer supplied from the power supply 4 to the load 3. Thus, the operation of the load 3 is controlled so that power supplied from the power supply 4 to the load 3 is adjusted by controlling the opening and closing operation of the relay 2 by the load control apparatus 101.
When oxide (silicon oxide) or carbide accumulates on the contacts of the relay, a failure such as contact failure (conduction failure) occurs.
For example, as described in Japanese Unexamined Patent Publication No. H4-216293, there is a method in which a relay operation detection circuit is provided for contacts of a relay, and a contact failure of the relay is detected by the relay operation detection circuit without operating a load.
Further, as described in Japanese Unexamined Patent Publication No. 2013-105550, there is a method in which as a measure for preventing a failure due to accumulation of oxide or carbide on contacts of a relay, the oxide or carbide accumulated on the contacts is cleaned off by an arc discharge generated during opening and closing of the relay by increasing a current flowing between the contacts of the relay or a voltage applied therebetween.
Further, for example, as described in Japanese Unexamined Patent Publication No. 2000-182495, as a cleaning method which does not use an arc discharge, there is a method in which when a contact failure is detected or when it is desired to clean contacts, the contacts are forcibly opened and closed and the contacts are cleaned by vibration or expulsion action due to the opening and closing operation.
As described above, a failure such as contact failure (conduction failure) caused by accumulation of oxide or carbide on the contacts of the relay causes a decrease in the rate of operation of the load control apparatus. Particularly, with a machine tool including a relay, there is a possibility that occurrence of such failure not only causes a decrease in the rate of operation but also leads to occurrence of a serious accident. Therefore, it is important to detect a relay failure.
According to the method described in Japanese Unexamined Patent Publication No. H4-216293, it is possible to detect a contact failure of the relay so that a serious accident may not occur due to a contact failure, but it is not possible to prevent a contact failure. Since oxide, carbide, etc., gradually accumulates on the contacts of the relay as time elapses, a contact failure of the relay is an unavoidable problem.
Further, according to the method described in Japanese Unexamined Patent Publication No. 2013-105550, oxide or carbide accumulated on the contact of the relay is cleaned off by an arc discharge, so that it is possible to prevent a contact failure due to accumulation of oxide or carbide. However, the arc discharge itself is a phenomenon that tends to damage the contact of the relay, and therefore, after all, occurrence of a failure such as decrease in life span and fusion of the contacts of the relay is unavoidable. Particularly, an arc discharge that occurs when the contacts of the relay are opened is larger in discharge energy and longer in duration than that which occurs when the contacts of the relay are closed, and therefore constitutes a major cause for the decrease in life span and fusion of the contacts.
Further, the method described in Japanese Unexamined Patent Publication No. 2000-182495 is for removing a deposit such as oxide or carbide by vibration due to the forced opening and closing operation of the contact of the relay without using an arc discharge. However, since the vibration due to the forced opening and closing operation of the contacts of the relay is small, elimination effect cannot be expected when the deposit to be eliminated or the contact itself are light-weight.